Secondary searches were underway, which could lead to a rise in the death and injury toll. Five people were confirmed dead several hours after the crash; a sixth died overnight, and another death was reported on Wednesday.

The derailment occurred at approximately 9:30 p.m. on the 2000 block of Wheatsheaf Lane in Philadelphia, according to officials, when six cars of Train 188 overturned in the city's Port Richmond section.

"We do not know what happened here. We do not know why this happened," Nutter said. Initial reports indicated that the train derailed as it entered a turn.

Nutter told CNN that there was no indication the derailment was a result of impact with another train.

"We were rolling along nice and smooth and then all of a sudden we were on our side," passenger Don Kelleher told WCAU.

Video from the scene showed emergency crews with flashlights swarming the crash site. The incident prompted a four-alarm response from approximately 120 firefighters and 200 police officers, according to WCAU.

Paul Cheung, director of interactive and digital news production for The Associated Press, was on the train and said it appeared as if it went off the tracks. "You could see everything starting to shake," he said. "You could see people's stuff flying over me."

The front of the train “looks pretty bad,” Cheung added. He said he saw "some people mangled up."

A passenger is carried following an Amtrak train crash near Philadelphia. (AP Photo/Paul Cheung)

Former U.S. congressman Patrick Murphy, who represented part of suburban Philadelphia, tweeted from the train, saying that he was not harmed, but others appeared injured.

"People were pretty banged up. There was a lot of blood, a lot of bleeding. I pulled myself up. The guy who I kind of landed on was OK. The guy next to him was completely passed out, knocked unconscious," Murphy told WPVI.

Sen. Tom Carper (D-Del.) was aboard the train, but had gotten off before the derailment.

"I am grateful to be home safe and sound in Wilmington, and my heart goes out to all those on the train tonight. I hope all of those that are injured recover quickly, and I will keep them in my thoughts and prayers," Carper said in a statement.

"My thoughts and prayers are with all of those impacted by tonight's train derailment," Wolf stated. "For those who lost their lives, those who were injured, and the families of all involved, this situation is devastating. I want to sincerely thank our first responders for their brave and quick action. I am closely monitoring the situation and I am in contact with state and local authorities."

Philadelphia Mayor Michael Nutter, center right, hugs Lori Dee Patterson, a nearby resident, after she handed him a cup of coffee after he spoke at a news conference near the scene of a deadly train derailment, Wednesday, May 13, 2015, in Philadelphia. An Amtrak train headed to New York City derailed and crashed in Philadelphia on Tuesday night, killing at least six people and injuring dozens more. (AP Photo/Matt Slocum)